Abstract: Soil organic matter is an important index to indicate ecological effects of grassland fencing. In this paper, soil organic matter fractions and carbon management index were studied by applying field investigation and laboratory analysis in semi-arid grasslands receiving different years of fencing. The response of soil organic matter to grassland fencing, which is essential for the improvement of soil quality and ecosystem rehabilitation, was analyzed. The results showed that the contents of soil organic matter and labile organic matter were decreased with soil depth, and were in order of 18 and 23 years fencing 13 years fencing unfencing treatment 3 years fencing. The carbon management index (CMI) of the soils received 3 years fencing was less than 100 in 0-90 cm soil profile, while the carbon management indices in 0-60 cm soils of 13 years fencing treatment, 0-90 cm soils of 18 and 23 years fencing were larger than 100, which indicated that the improvement of soil organic matter by fencing are more deeper in soil profiles with the increase of fencing age. The three labile organic matters were significantly related with soil organic matter and most soil properties, suggesting that labile soil organic matters could sensitively and directly indicate long term effects and soil quality changes with land/soil management.